1. Field of the Invention
The present invention relates to a playback apparatus, a playback method, a recording medium, and a program. More particularly, the present invention relates to a playback apparatus for playing back a moving image, a playback method therefor, a recording medium therefor, and a program therefor.
2. Description of the Related Art
As a system for recording image data as a digital image signal on a DVD (Digital Versatile Disc) and a CD (Compact Disc), an MPEG (Moving Pictures Experts Group) system is widely used. In the MPEG system, an I (Intra coded) picture, a B (Bidirectionally predictive-coded) picture, and a P (Predictive coded) picture, which are generated by using a compression technology called an “inter-frame prediction”, are defined. Data of an I picture, data of a P picture, and data of a B picture are image data that is compressed and coded in such a manner that data of an I picture is generated independently of other frames, data of a P picture is generated from past data of an I picture or a P picture, and data of a B picture is generated from past and/or future data of an I picture or a P picture.
In the image data of the MPEG system, which is recorded on a DVD or a CD, as shown in FIG. 1, an I picture, a B picture, and a P picture are arranged in such a manner that a fixed arrangement order is repeated. A GOP (Group of Pictures) described in the MPEG is a fixed arrangement order of an I picture, a B picture, and a P picture that is repeated, and is the unit of coding. One GOP contains at least one I picture. That is, data of an I picture, which is a coding image plane by closed information within the image plane, is periodically inserted into image data. The term “GOP” refers to an image plane group structure in which at least one I picture is contained.
The I picture, the B picture, and the P picture will be hereinafter referred to simply as a “picture” when these do not have to be distinguished individually from one another.
FIG. 2 shows the configuration of a playback apparatus of the related art, for playing back recorded image data. A DVD 1 has recorded thereon image data of the MPEG system. A drive 2 reads data of a picture among the recorded image data of the MPEG system. A storage control section 3 controls the storage of a buffer 4 for temporarily storing read data of a picture forming the image data.
A decoder 5 sequentially reads the stored data of a picture from the buffer 4, decodes the read data of a picture, and supplies so-called baseband image data, which is obtained by decoding and which is not compression-coded, to a display control section 6 at an interval in accordance with the frame rate (video frame rate).
The display control section 6 causes a display device 7 to display a moving image on the basis of the image data supplied from the decoder 5 at a predetermined frame rate.
In this manner, a moving image is displayed at a normal speed of the moving image using the image data recorded on the DVD 1.
In the playback of a moving image at a normal speed of the moving image, the storage control section 3 controls the storage of data of a picture into the buffer 4 so that the moving image displayed on the display device 7 is not interrupted and an overflow or an underflow does not occur in the data of a picture, which is recorded in the buffer 4. As shown in FIG. 3, when the amount of the data of a picture stored in the buffer 4 exceeds a threshold value for detecting an overflow, the storage control section 3 controls the storage so as the drive 2 to stop the reading of the data of a picture from the DVD 1, so that an overflow does not occur in the buffer 4.
When the decoder 5 is made to continue the decoding while the drive 2 keeps stopping the reading of data of a picture from the DVD 1, the amount of the data of a picture stored in the buffer 4 decreases. When the amount of the data of a picture stored in the buffer 4 falls below (becomes less than) the threshold value for detecting an underflow, the storage control section 3 controls the storage so that the storage control section 3 causes the drive 2 to restart the reading of the data of a picture from the DVD 1 so that an underflow does not occur in the buffer 4.
When attempts are made to perform a so-called high-speed playback for playing back a moving image at a speed n times as fast as the normal speed of the moving image by the same method as the method of playing back a moving image at the normal speed of the moving image, there are cases in which it is difficult for the decoder 5 to decode the data of a picture within the requested time, and it is difficult to play back the moving image at a target speed.
For this reason, as shown in FIG. 4, there is a known method of continuing a high-speed playback by decoding only the data of a picture at the beginning among the I picture, the P picture, and the B picture contained in one GOP. That is, the drive 2 reads, from the DVD 1, only the data of an I picture at the beginning among the I picture, the P picture, and the B picture contained in one GOP, and stores the read data of an I picture in the buffer 4. Next, the data of an I picture stored in the buffer 4 is sequentially transferred to the decoder 5, and the decoder 5 decodes the data of an I picture. The decoder 5 supplies the baseband image data obtained as a result of the decoding to the display control section 6. The display control section 6 displays the image on a display device on the basis of the baseband image data.
As described above, it is possible to play back a moving image at a speed n times as fast as the normal speed of the moving image. The reason why only the data of an I picture is used in the high-speed playback is that the data of an I picture can be decoded singly because the data of an I picture, unlike the P picture or the B picture, has no correlation with the other pictures.
Here, if the number of pictures contained in one GOP is denoted as Np, the speed at which a playback can be performed in the high-speed playback method of playing back only the I picture at the beginning contained in one GOP is Np times as fast. That is, when the number of pictures Np contained in one GOP is 6, while 6 pictures contained in one GOP are played back in the playback at a normal speed, the I pictures of 6 GOPs are played back in the high-speed playback, and the playback speed in the high-speed playback becomes 6 times as fast.
In the above-described high-speed playback method for playing back only the I picture at the beginning contained in one GOP, the playback speed is determined by the number of pictures Np, which is the number of pictures contained in one GOP, and it is difficult to change the playback speed.
Therefore, in order to increase the degree of freedom of the speed in the high-speed playback, a high-speed playback method may be used in which, rather than playing back the I pictures at the beginning of all the GOPs, the GOPs are skipped so that only the one I picture is played back from one GOP for each skip interval Ns, which is a fixed interval, and the same I picture is continued to be displayed repeatedly by the number of consecutive frames Nh, which is a fixed number of frames.
In this high-speed playback method, the playback speed is determined by the skip interval Ns, the number of consecutive frames Nh, and the number of pictures Np. That is, the moving image is played back at a speed determined by Ns×Np/Nh. Therefore, as a result of adjusting the skip interval Ns and the number of consecutive frames Nh, the moving image can be displayed at a desired playback speed without being limited to the number of pictures Np.
In the high-speed playback method in which the GOPs are skipped so that only the one I picture is played back from one GOP for each skip interval Ns, which is a fixed interval, and the same I picture is continued to be displayed repeatedly by the number of consecutive frames Nh, which is a fixed number of frames, a plurality of combinations of the skip interval Ns and the number of consecutive frames Nh for performing a high-speed playback at a desired speed exist.
FIG. 5 shows the relationship between GOPs and the skip interval Ns and between frames and the number of consecutive frames Nh. In the case shown in FIG. 5, since a high-speed playback is performed on the basis of the skip interval Ns of 3 and the number of consecutive frames Nh of 2, one I picture is extracted from 3 GOPs, and the I picture is displayed as a frame for two times. In the case shown in FIG. 5, since one GOP is composed of 6 pictures, the playback speed becomes Ns×Np/Nh, that is, 3×6/2=9 times as fast.
However, it is difficult to determine the combination of the skip interval Ns and the number of consecutive frames Nh in the high-speed playback at a desired speed as desired without considering constraint conditions at all.
For example, there are two constraint conditions described below. A first constraint condition is concerned with influences upon a user. That is, it may be said that, when the number of consecutive frames Nh is great, the same image is displayed in a continued manner, and the greater the number of consecutive frames Nh, the more information is lost from the moving image displayed on the display device 7. That is, it may be said that the greater the number of consecutive frames Nh, the more information to be provided to the user is lost. For example, when the number of consecutive frames Nh is too great, when the user wants to find a target scene, the image of that scene is not displayed on the display device 7 and the user is unable to find the scene. Furthermore, when the number of consecutive frames Nh is too great, the user is forced to view the same image for a long period of time. That is, the user feels in such a manner that a still image is switched for each that period and it is difficult for the user to obtain a sense of reality of performing a high-speed playback.
A second constraint condition is concerned with the performance of the playback system, which is related to the reading and decoding of data.
More specifically, the data of an I picture has a larger amount of data in comparison with the data of a P picture or the data of a B picture. Therefore, if the skip interval Ns is too small, the amount of data that is requested in the high-speed playback per unit time, becomes sharply increased in comparison with the amount of data that is requested in the normal playback per unit time.
Furthermore, when the skip interval Ns is small, the positions (intervals) on the DVD 1 on which the data of an I picture to be read from the DVD 1 is recorded become small. When the next data of an I picture is to be read after one piece of the data of an I picture, the position of the next data of an I picture is gone too far, the drive 2 needs to seek or needs a rotational delay, and thus, there may need to take a longer time to read the data of an I picture. In such a case, when compared to the amount of the requested data of an I picture per unit time, the amount of the data of an I picture transferred actually to the buffer 4 per unit time becomes smaller.
Since the reading from the buffer 4 can be performed at a high speed, if the skip interval Ns is too small, the transfer rate at which the data of an I picture is read from the DVD 1 and is transferred to the buffer 4 falls below the rate at which data is transferred from the buffer 4 to the decoder 5. The term “transfer rate” as used herein refers to the amount of data transferred per unit time.
Then, a state is reached in which the amount of the data stored in the buffer 4 typically falls below the threshold value for detecting an underflow, and it is difficult to perform a playback.
Therefore, it is necessary that the transfer rate at which the data of an I picture is read from the DVD 1 and is transferred to the buffer 4 does not fall below the rate at which data is transferred from the buffer 4 to the decoder 5.
Hitherto, there is a known method in which, when a 3× speed playback is performed, a repeated playback of an I picture that is extracted from within information in VOBU (Video Object Unit) units and that is decoded by 15/3=5 times (5 frames) is repeated in VOBU units, and when a 5× speed playback is performed, a repeated playback of an I picture that is extracted from within information in VOBU units and is decoded by 15/5=3 times (3 frames) is repeated in VOBU units (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-312663).